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This invention relates to the field of electronic circuit board manufacture. More particularly, this invention relates to an improved system for inspecting solder paste deposited on a circuit board during manufacture.
Circuit boards that carry electronic integrated circuits as well as discrete electronic components are well known. A circuit board substrate is prepared with predetermined conductor paths and pads for receiving the lead of an electronic component such as integrated circuit chips, resistors or capacitors. During the circuit board fabrication process, solder paste bricks are placed onto the board substrate at appropriate positions. The solder paste is usually applied by placing a screen onto the substrate, applying solder paste through the screen openings and removing the screen from the substrate. The circuit board electronic components are then positioned onto the substrate, preferably with a pick and place machine, with leads of the electronic components placed on the respective solder paste bricks. The circuit board is passed through an oven after all of the components are positioned on a substrate to melt the solder paste thus creating an electrical as well as mechanical connection between the components and the substrate.
The size of the solder paste bricks and the accuracy with which they must be placed on the substrate has become increasing smaller and tighter with the increased emphasis on miniaturization in the electronics industry. Solder paste brick heights can be as small as 100 microns and the height of the solder paste brick must often be measured to within 1 percent of the designed height and size. The center-to-center spacing between solder bricks is sometimes 200 microns. Too little solder paste can result in no electrical connection between the lead of an electronic component and the pad of the circuit board substrate. Too much paste can result in bridging and short-circuiting between the leads of a component.
A single circuit board can cost thousands and even tens of thousands of dollars to manufacture. Testing of a circuit board after the fabrication process is complete can detect errors in solder paste placement and component lead connection, but often the only remedy for a faulty board is rejection of the entire board. It is accordingly imperative that a circuit board be inspected during the fabrication process so that improper solder paste placement can be detected prior to the placement of the electronic components onto the substrate. Such in-process solder inspection reduces the cost of failure since expensive components have not yet been placed onto the circuit board.
Current solder paste inspection systems have a number of limitations. First, such systems are typically costly and the cost of the system must be borne by board manufacturing prices, and thus finished circuit board prices. Further, current systems are relatively inflexible when tasked with inspecting features of different levels of detail. Such current systems are unable to measure coarse objects with the same sensor, or optical train, as fine objects. Thus, two different optical trains of differing capabilities are required to provide two levels of resolution and throughput, adding significantly to system cost.
Another limitation of current solder paste inspection systems is their susceptibility to vibration. In these systems, the target must remain in a known position in order to achieve high accuracy measurements. Vibration causes features on the target surface to appear in different places than expected. Such spatial error adversely affects the accuracy and repeatability of solder paste height, volume and area measurements, and can lead to the acceptance of a defective solder brick, or the rejection of an otherwise acceptable brick.
Another limitation of current inspection systems is that of inspection speed. Since in-process solder paste inspection is generally performed in the assembly line, the inspection step itself must be done as quickly as possible in order to minimize the impact of inspection on the assembly line""s throughput.
User-friendliness is another limitation of current systems. One example of this limitation is that current solder paste inspection systems require users to program a number of points used by the system to establish a reference plane for solder paste height and volume calculation. Eliminating such user setup would significantly facilitate user interaction with the inspection system.
Finally, traditional solder paste inspection systems use relatively high powered lasers (Class III) for imaging. For safety, the FDA has placed strict safety precautions upon the use of such lasers. These precautions can render use of such devices cumbersome. Thus, there is a current need to provide a solder paste inspection system that does not employ any Class III lasers whatsoever.
A novel inspection system for inspecting articles of manufacture, such as a printed circuit board is disclosed, where the system includes a strobed illuminator adapted to project light through a reticle so as to project a pattern of light onto an area of the printed circuit board. A transport mechanism responsively positions the board to at least two distinct positions, where each position corresponding to a different phase of the projected light. Also included is a detector adapted to acquire at least two images of the area, each image corresponding to one of the at least two different phases. A position encoder monitors the movement of the board and outputs a position output, and a processor connected to the encoder, the transport mechanism, the strobed illuminator and the detector controlledly energizes the strobed illuminator to expose the area as a function of the position output, the processor co-siting the at least two images and constructing a height map image with the co-sited images. In a preferred embodiment of the present invention and method, the detector is further adapted to acquire an additional image of the area as a function of a position of the target, and the processor operates upon three images of the area to provide the compensated height map. In another embodiment, the illuminator is strobed at least two times within a short period of time, such as two milliseconds, to reduce vibration sensitivity of the system. In another embodiment of the invention, solder paste volume is computed from the acquired images and the height map. Another embodiment provides a low resolution, high speed mode where charge from a plurality of pixels is combined to form a larger effective pixel, for use in high speed applications. Optionally, the height map is de-tilted in order to compensate for physically or algorithmically tilted targets.